Transcript Loc TEM

Network Solutions Sector
TSG-RAN Working Group1 meeting#2
TSGR1#2(99)081
Yokohama 22-25, February 1999
Agenda Item:11
Source: Motorola
Title: Shared Channels for Packet
Transmission in W-CDMA (Slides)
Data Document for: Other Business
1
Network Solutions Sector
SHARED CHANNELS FOR PACKET DATA
TRANSMISSION IN W-CDMA
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Network Solutions Sector
Outline
• Introduction
• Strategy
• UMTS Packet Data Implementation
• Advantages of Shared Channel
• Benefits of Fat Pipe
• Downlink Shared Channel (DSCH)
• Limitations of Packet Modeling Techniques
• Uplink Shared Channel (USCH)
• Conclusions and Recommendations
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Network Solutions Sector
Don’t Transmit Packets on Circuits
• Current UMTS approach looks more like fast circuit than packet
switching.
– For short packets, RACH is used.
– For long packets, RACH sets up a brief circuit connection
• Resource requirements changing continuously.
– Not possible to negotiate appropriate data rate a priori.
– Data rate is determined by Packet Size X Interarrival Time.
– UTRAN must estimate the source data rate based on packet arrivals.
• Internet/Intranet will be terminus for most data services.
– Employ common IP packet scheduling.
• “Random Early Detection” (RED) for congestion avoidance
• “Weighted Fair Queuing” for packet scheduling
– Adopting IP Techniques will insure compatibility with new Internet
applications.
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Network Solutions Sector
Strategy
• Interference Management for Packet Channels
– Provide uniform composite interference of all packet users across the cell
– Schedule packet data burst intelligently to satisfy power and interference
constraints of the cell in question
• Maximize statistical multiplexing gain
– Maximize peak transfer rates to a single mobile
• Allocate a high rate channel to a single user rather than multiple low rate
channels to multiple users
– Minimize the access and paging delay for quick allocation of resources
– Efficiently multiplex small packets from/to multiple mobiles
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Network Solutions Sector
UMTS Packet Data Implementation
• Shared Channel maximizes statistical multiplexing gain
– Assign the fattest possible data pipe to a user so that overall delay experienced
is minimized
• Downlink Shared Channel (DSCH)
– Power and code resource is shared between users
– Overcomes the problem of downlink OVSF code shortage
• Uplink Shared Channel (USCH)
– Limited power resource which is shared between users
– Problem of code shortage does not exist
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Network Solutions Sector
Advantages Of Shared Channel
• Advantages of Shared Channel over Dedicated Channels (DCH’s) controlled
by RRC
– Resource more fully used in every frame ( provided there are packets to transmit)
– Facilitates efficient shared access to a large data pipe
• Highest priority packets gets served first, irrespective of which UE the packets are
going to/from. This improves QoS.
• Average packet call completion times improved.
– The data rate of the shared channel can be dynamically varied in response to
rapid change in conditions.
– No reliance on imperfect packet call admission control which with DCH approach
can result in inappropriate data rate assignment.
– For the case of downlink
• Shared channel provides an efficient method to access limited downlink OVSF
codes
• Proportion of power assigned for carrying packet connections could be packed
more efficiently when shared channel is used
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Network Solutions Sector
MAC Scheduling at the CRNC
• Perform MAC scheduling in the CRNC on shared channels as
opposed to RRC scheduling at the SRNC onto DCH’s
• By only making short leases on the radio resource a light-weight
protocol can be exploited
• Perform scheduling on MAC instead of RRC in order to minimise
signaling and processing overhead
• Enable CRNC to perform scheduling (as opposed to SRNC) in order
to reduce message exchanges across Iur and to thereby facilitate
fast scheduling onto the fat pipe
• Resource Allocations for each frame are signaled in each frame
– Therefore no need for acknowledged mode signaling
– More efficient resource usage, improved packet call completion times
– Faster scheduling
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Benefits of Fat Pipe
Network Solutions Sector
• Findings published in Motorola Contribution to SMG2 UMTS-L23
534/98 dated 12/9/98
• Preferable to allocate the total packet bandwidth allocation to a
single user than to allocate an equal total packet bandwidth of
multiple narrower band channels to simultaneous users.
Models for various services.
Service
Session
arrival
Ave # of
pkt calls
per
session
Ave
reading
time
between
pkt calls
Ave
number of
pkts in pkt
call
Ave inter- Packet
arrival
sizes
time
between
pkts
web
Poisson
5[5]
120 sec[8]
25[5]
10 msec
480[5]
email
Poisson
1
--
15[9]
10 msec
158[9]
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Benefits of Fat Pipe (cont’d)
Network Solutions Sector
Simulation results for single service (Web Browsing) implementation.
Web Browsing
Number of
packet
channels
Pkt Ch
BW
Kbps
1
8
16
307
38.6
19.2
Percent load
Average
Queue delay
(seconds)
Average
Transmission
time (second)
Total delay
time
(seconds)
75
75
75
1.27
0.708
0.461
0.313
2.51
5.02
1.58
3.22
5.48
Average
Transmission
time (second)
Total delay
time
(seconds)
Simulation results for single service (Email) implementation.
Email
Number of
packet
channels
Pkt Ch
BW
Kbps
Percent load
Average
Queue delay
(seconds)
1
307
75
0.24
0.049
0.289
8
38.6
75
0.088
0.39
0.478
16
19.2
75
0.05
0.788
0.838
Note: Total delay time in Table 1 and 2 refer to packet call completion time
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Network Solutions Sector
Overview of Downlink Shared Channel
(DSCH)
• Two methods for DSCH have been proposed
– DSCH with Time Multiplexed Packet Users (proposed by Lucent, Sony
and Nortel, Tdoc SMG2 UMTS-L23 159/98, 320/98, 266/98, 169/98)
– DSCH with Fast Code Multiplexing (proposed by Nokia and Motorola,
Tdoc SMG2 UMTS-L23 296/98, 533/98)
• It was agreed in SMG2 that DSCH should utilize Fast Code
Multiplexing (FCM).
• Concept of DSCH included in ETSI’s document#XX03-130
• Two possibilities exists for carrying the control information for
DSCH
– Using a dedicated channel (DCH)
– Using a common DSCH control channel (also called the ACCH)
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Network Solutions Sector
DSCH with Fast Code Multiplexing
• Segment of the Code Tree for Orthogonal Variable Spread Factor
(OVSF) codes assigned to packet data services
• The number of OVSF codes assigned for packet data services (and
the number of UE’s served) can change on a frame by frame basis
16
8
4
17
9
32 = No code
2
5
10
11
Tree Access
Point
23
24
1
12
SF=8
6
25
26
13
3
27
28
7
SF=16
14
29
30
15
Branch of code tree assigned to
packet data services.
18
19
20
21
22
SF=32
SF=64
31
SF=128
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Code Assignment for the DCH
Network Solutions Sector
• A 384 kbps packet data service is assigned a SF = 8.
• Seven 384 Kbps UEs at activity rate of 1/10 consume 87% of OVSF
tree.
SF=1
SF=2
SF=4
SF=8
SF=16
SF=32
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Code Assignment for the DSCH
Network Solutions Sector
• All 384 kbps on the DSCH monitor the same ACCH at SF=64 and the
SF=8 is assigned as needed.
• The same seven 384 kbps UEs at activity rate of 1/10 consume only
14% of OVSF tree.
SF=1
SF=2
SF=4
SF=8
SF=16
SF=32
SF=64
SF=128,256
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Network Solutions Sector
Signaling Options for DSCH
• DSCH is associated with a DCH
– Disadvantages
• Less powerful coding on allocation messages (e.g. (32,6) Bi-orthogonal
Code used for TFCI field)
• Signaling resources consumed will be proportional to the number of users
• DSCH is associated with a common control channel called Access
Control Channel (ACCH)
– Advantages
• ACCH time multiplexes all assignments on a single, relatively low rate,
OVSF code, thus reducing the overall OVSF codes used for control
• ACCH is always synchronized to the frame timing of the current cell
– Disadvantage
• Fixed power allocation, does not use Fast Forward Power Control (FFPC)
– Simulations show that ACCH will be more efficient when resources are
needed the most.
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DSCH control channel efficiency
Network Solutions Sector
Probability of N or more Simultaneous Packet Calls
Dedicated Channel
(Web Browsing)
more efficient
100%
90%
Common Channel
more efficient
80%
75%
90%
92%
95%
70%
60%
50%
40%
30%
20%
10%
0%
0
10
20
N
30
40
50
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Common Control Channel vs. Dedicated
Control Channel
Network Solutions Sector
Shared Channel
Utilization
Session
Arrival Rate
Mean Simultaneous Users
Likelihood a Common Channel
will be more Efficient
Analytic
Simulated
10:1
20:1
95%
0.76
18.05
16.5
57%
34%
92%
0.72
10.58
10.9
42%
20%
90%
0.70
8.1
7.86
31%
10%
70%
0.60
2.25
2.19
7%
0.3%
• Summary
– For low channel utilization DCH is efficient
– For high channel utilization or when resources are needed most ACCH is
more efficient
• Recommendation
– Provide both methods in the specification
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Network Solutions Sector
•
Limitations of Packet Modeling
Techniques
Number of simultaneous users is sensitive to packet interarrival time.
– Congestion elsewhere in the network may increase interarrival times
– Confidence in the data models is modest at best?
• Will all applications fit into the narrow data models for ftp, www, and email?
• What are the correct proportions?
•
UMTS protocol must adapt to data traffic presented. A Common Control
Channel makes no assumptions on data traffic patterns.
•
Maximum packet size is governed by the IP Maximum Transmission Unit
(MTU).
– Typical MTU is on the order of 500 bytes.
– 1500 bytes is the practical maximum for the MTU
– ETSI’s model specifies a maximum of 66,000 bytes
•
The total data transfers sizes will not be known a priori. Therefore, the
dedicated channel may not be as effective as previous simulations suggest.
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Network Solutions Sector
Details of the Common Control
Channel for DSCH
• Common Control Channel for DSCH (Access Control Channel (ACCH))
– Aggregates functions of
• Uplink Power Control
• Dynamic Persistence for RACH
• Downlink OVSF Code Assignment
• Uplink SF Assignment
• Uplink Timing Event
– ACCH provides a direct method for assigning resources of the shared
channel
– ACCH is not power controlled
– ACCH is transmitted over the entire cell
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Structure of the ACCH
Network Solutions Sector
NPilot
NTPC
NData
Pilot
TPC TPC TPC TPC TPC TPC TPC TPC
#0
#1
#2
#3
#4
#5
#6
#7
Coded Assignment Information
0.625 ms, 20*2k bits (k=0..6)
Slot #1
Slot #2
Slot #i
Slot #16
Tf = 10 ms
Frame #1 Frame #2
Frame #i
Frame #72
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Network Solutions Sector
ACCH Assignment Fields
Downlink Shared Channel Assignement
Field
Bits
TUEID
8
OVSF Code
Assignment
Total
7
Reference
A 8-bit temporary ID providing unique to particular
cell
Assigns a specific branch of the code tree.
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Uplink Shared Channel Assignment
Field
Bits
Reference
TUEID
8
A 8-bit temporary ID providing unique to particular
cell
SFA
3
Assigns the spreading factor for the next frame.
PCPA
3
Assigns a position in the common power control
channel for the duration of the transfer.
Total
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Number of Assignments per Frame for
Various SF and Coding Rates
Network Solutions Sector
Spreading
Factor
Npilot
NTPC
256
128
64
128
8
8
8
8
8
8
8
8
Ndata
Frame
Period
(ms)
Repitition
Coding
CRC
Tail
Info bits
available
DPI
4
24
64
24
10
10
10
10
1
3
1
0
1/3
1/3
1/3
1/2
16
16
16
16
8
8
8
8
-3
103
317
168
10
10
10
10
DOCA/user UPA/user
15
15
15
15
14
14
14
14
Users
0
3
10
5
• With a Spreading Factor of 128 and using R=1/2 Convolutional or Turbo
Code, ACCH can accommodate assignments for 5 UE’s in both direction or
assignments for 10 UE’s in one direction simultaneously per frame.
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Network Solutions Sector
Overview of Uplink Shared Channel (USCH)
• USCH represents a shared power resource
• USCH coordinates fast scheduling of uplink data packets
– Insure an uniform interference power profile protecting voice users
– Schedule “Budgeted Noise Rise”
• Each active MS is assigned a fraction of total noise rise which translates to a
Spread Factor (SF) assignment
• Reassign the data rate on a frame by frame basis (functionally equivalent to
downlink FCM)
• UE synchronizes framing to the strongest BTS on the active set
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Network Solutions Sector
USCH Details
• Commonality with DCH
– Identical PDTCH channel frame formats
– Ability to perform fast and slow power control
– May employ soft handoff if necessary
• Differences from the DCH
– Discontinuous uplink transmission requires a one frame preamble
before the start of data transmission.
– Performance is identical to DCH when frames are consecutive
– The preamble will prime acquisition, channel estimation and
power control.
– Timing advance or guard band is required for large cell sizes
– Transmission from an near to BTS UE may overlap the
transmission from a far from BTS UE, resulting in excessive noise
rise.
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Network Solutions Sector
Fast Power Control and Channel
Estimation for USCH
• Convergence of power control loop and the availability of good
channel estimates are critical for operation of USCH. Two solutions
are envisaged
• Use of a low rate bi-directional link maintenance channel between
packet burst
• Unnecessary power resource is consumed when there are no packets
to transmit
• Increase in uplink noise rise
• Maintaining a dedicated downlink channel for each uplink channel will
worsen the code shortage problem
– Preamble transmission using DPCCH before packet data transmission
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Network Solutions Sector
Downlink Physical
Channels
Bi-directional Link Maintenance Channel
10 ms
ACCH
DSCH
PRACH
DPCCH
USCH
Uplink Physical
Channels
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Preamble Transmission
Network Solutions Sector
Downlink Physical
Channels
10 ms
ACCH
DSCH
PRACH
DPCCH
USCH
Uplink Physical
Channels
Ideal P
ower S
ett
ing
1dB
Open Loop Power Estimate
2 dB
X dB
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Network Solutions Sector
Preamble Transmission (cont’d)
• Three cases are considered in the Figure
– No need for Preamble, if RACH is used before transmission of packets
– Preamble used to converge uplink DPCCH (for power control, channel
estimation and acquisition), before packet data transmission starts on
DSCH
– Preamble used to converge uplink DPCCH (for power control, channel
estimation and acquisition), before packet data transmission starts on
USCH
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Preamble Transmission
Network Solutions Sector
Ad
Cd
Switched off during
preamble transmission
DPDCH
Cscramb
QPSK
Modulation
Ap
DPCCH
Cc
j
29
Network Solutions Sector
Consecutive Idle Frames within a Packet Call
for Various Values of System Utilization
Mean packet size = 480 bytes
Rate = 384 Kbps
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Network Solutions Sector
Timing Events
• If a far-end UE and a near-end UE is assigned a low SF code in
consecutive frames, the last part of transmission from far-end UE
may collide with the first part of transmission from near-end UE (due
to propagation delay) resulting in excessive noise-rise in the cell in
question.
• UE’s need to retard their timing by an amount Dt to prevent collisions
•
Three methods are proposed for computation of Dt:
– Method1 - Dt is computed based on a relative distance between the two
UE’s w.r.t BTS
– Method2 - Dt is computed based on a distance between a single UE and
the BTS
– Method 3 - Uses a fixed guard period
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Network Solutions Sector
UE’s Propagation Delay w/o Timing Offset
Correction
UE A
UE B
Node B
UE C

UE A Transmit Timing

UE B Transmit Timing

UE C Transmit Timing
•  is proportional to the range between the node B and UE#A
•  is proportional to the range between the node B and UE#B
•  is proportional to the range between the node B and UE#C
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Signaling Methods for Timing Events
Network Solutions Sector
Frame #1
Frame #2
Frame #3
Frame #4
Frame #5
Dt4
Dt5
Frame #6
Frame #7
Frame #8
Frame #9
Dt7
Dt8
Dt9
10 ms
ACCH
UE A
UE B
UE C
Dt1
Dt2
Dt6
Dt3
Uplink Shared
Channel
• Method - 1
– TOA from UE#A to node-B - l
– TOA from UE#B to node-B - m
– UE#B retards its frame timing by an amount Dt2 = l-m
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Network Solutions Sector
Signaling Methods for Timing Events
(cont’d)
• Method - 2
– At Frame#1 an offset Dt2 is broadcast using ACCH
– UE#B transmits data packets using an offset Dt2
– TOA denoted by r between the Node B and UE#B is computed
– Node B broadcasts offset Dt3 = Dt2 + using ACCH
– UE#C transmits data packets using an offset Dt3
– Offset is reset after it reaches a set threshold e.g. 10000 ms
• Method - 3
– UE’s uses a fixed guard period n (set to 100 ms for cell size of 16 km)
– Dt3 = Dt2 +n
– Dt4 = Dt3 +n
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Network Solutions Sector
QoS for W-CDMA Packet
• Base QoS on network and application standards
– Internet QoS (End-to-end QoS support)
• Guaranteed throughput and bounded delays
• FER is irrelevant for most or manydata services, networks are effectively
perfect. However, delay is related to operating FER.
– QoS negotiation (analogous to call set-up)
• Admission control for premium service levels
• At L2 & MAC each mobile has QoS associated
• Implications for the MAC scheduling
– Need to signal multiple queue depths (per QoS level) during RACH
– Scheduling based QoS level
– May police mobiles with respect to negotiated QoS.
– Must standardize method for representing mobile QoS with UTRAN.
35
Network Solutions Sector
Conclusions
• Shared Channel maximizes statistical multiplexing gain
• Resource fully used in every frame
• Problem of downlink code shortage is mitigated using DSCH with
Fast Code Multiplexing (FCM)
• Limited power resource is shared between users using USCH
• Provides fast power control, transmit diversity and soft-handoff
• Recommendations for 3GPP specification:
– DSCH and USCH
– Provisions for DSCH and USCH to be associated with ACCH
– Provisions for DSCH and USCH to be associated with DCH
– Provisions for Preamble based transmission for uplink
– Provisions for Link Maintenance for uplink
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